I. Field of the Invention
This invention relates generally to a cancer therapy and, more particularly, to a medical particle delivery system having an achromatic and uncoupled gantry design.
II. Background of the Related Art
In traditional radiation therapy, X-ray beams are typically used to treat cancer. However, X-rays release much of their energy quickly after penetrating the skin, disrupting the molecules of healthy tissue and organs. Protons, neutron, α-ray or other ion rays, on the other hand, have excellent physical properties for radiation therapy which permit one to control very precisely the shape of the dose distribution inside the patient's body. The dose delivered by such an ion ray beam is well localized in space, not only in the lateral direction, but also very precisely in depth, due to the presence of the characteristic Bragg peak. Thus, ion ray therapy is effective because of its ability to accurately target and kill tumors, both near the surface and deep seated within the body, while minimizing damage to the surrounding tissues. For this reason, it is favored for treating certain kinds of tumors where conventional X-ray and radiation oncology would damage surrounding tissues to an unacceptable level.
It has been known in the art to use a particle accelerator, such as a synchrotron, and a gantry arrangement to deliver a beam of ion particles from a single source to one of a plurality of patient treatment stations for cancer therapy. Such cancer treatment facilities are widely known throughout the world. For example, U.S. Pat. No. 4,870,287 to Cole et al. discloses a multi-station proton beam therapy system for selectively generating and transporting proton beams from a single proton source and accelerator to one of a plurality of patient treatment stations each having a rotatable gantry for delivering the proton beams at different angles to the patients.
The beam delivery portion of the Cole et al. system includes a switchyard and gantry arrangement. The switchyard utilizes switching magnets that selectively direct the proton beam to the desired patient treatment station. Each patient treatment station includes a gantry having an arrangement of three bending dipole magnets and two focusing quadrupole magnets between each set of bending dipole magnets. The gantry is fully rotatable about a given axis so that the proton beam may be delivered at any desired angle to the patient located at the isocenter of the gantry. The gantry of typical particle beam cancer therapy systems accepts a particle beam of a required energy from the accelerator and projects it with a high precision toward a cancerous tumor within a patient. The beam from the gantry must be angularly adjustable so that the beam can be directed into the patient from above and all sides.
The disadvantage of such a gantry arrangement, however, that if the non-symmetric ion beam (i.e., a beam having different emittances in vertical and horizontal planes) is introduced into the gantry from a fixed transfer line, the beam transport within the gantry arrangement of Cole et al. becomes dependent on the angle of gantry rotation, which means that the patient will not receive the same high-precision beam spot from every direction.
In order to circumvent the disadvantage of the Cole et al. system, it has been proposed to include within a gantry setup a collimator, a special device that narrows a beam by filtering the beam particles so that only the rays traveling parallel to a specified direction are allowed through. Naturally the drawback of using such a device is a significant beam intensity loss and/or continuous beam tuning, which may require additional unnecessary time during the irradiation of the patient.
Benedikt, et al., on the other hand, proposed to use a special matching section, called a “rotator,” which in essence a plurality of quadrupole magnets positioned just in front of the gantry present in addition to the quadrupole magnets within the gantry. (M. Benedikt and C. Carli, “Matching to gantries for medical synchrotrons”, Particle Accelerator Conference PAC '97, Vancouver 1997). The rotator allows for the section of the beam line just before the gantry to be synchronously rotated in proportion to the gantry rotation. However, the disadvantage of the Benedikt et al. system is that it occupies about 10 m of extra length of the transfer line and requires an extra equipment for extremely precise mechanical rotation, which is a significant drawback for design of compact medical accelerator complexes appropriate for use in the hospital facilities.
Yet another approach to overcome beam dependence on the angle of gantry rotation was proposed by Dolinskii and disclosed in the U.S. Pat. No. 6,476,403. The gantry design of Dolinskii is based on a plurality of quadrupoles that create a fully achromatic beam transport, which is independent of gantry rotation. Nonetheless, the drawback of such a system is that the beam at the entrance of the gantry must be constrained to have the same angular divergence or size in the horizontal and vertical planes, which requires additional system to control the beam itself.
Accordingly, it would be desirable to focus the beam in such a way that the focusing of the beam at the exit of the gantry is always independent of the rotation angle of the gantry, thus, avoiding the use of collimators, rotators, or additional equipment to control the beam divergence, which may cause beam intensity loss or additional time in irradiation of the patient, or disadvantageously increase the overall gantry size inapplicable for the use in the medical treatment facility.